Natural killer (NK) cells are lymphocytes that provide innate immunity against infection, co-operate with dendritic cells to initiate adaptive immunity and remodel uterine tissue in reproduction. Their responses are shaped by a battery of activating and inhibitory cell-surface receptors. In man the most variable of these are the killer cell immunoglobulin-like receptors (KIR). The KIR family has both activating and inhibitory members. Inhibitory KIR ligands have been shown to be HLA class I determinants, but the ligands of activating KIR, which comprise almost half of the human KIR repertoire, are largely unknown. In the previous funding period we examined the natural history of the activating receptors including both KIR and their functional analog in rodents Ly49. This analysis yielded a two stage model for activating KIR evolution. In stage one, creation of an activating receptor is followed by rapid increase in frequency due to beneficial effects. When the selective advantage of the activating receptor disappears, it enters stage two where its deleterious effects (e.g. autoimmunity) lead to decreasing frequency and eventual gene loss. This model is in agreement with correlations of activating KIR with autoimmune disease progression. That human activating KIR are in stage two of our model is indicated by their frequency as well as the results of a selection analysis that indicated activating KIR specific residues, close or in the predicted HLA binding site, that appeared to be under positive selection. Our model predicts that receptors in stage two of the model will exhibit decreased binding to their ligands. We will test this prediction by using an in vitro binding assay to assess the binding profiles and affinities of the activating KIR. It is unclear whether KIR2DS4 is in stage one or stage two of our model and the second part of Aim 1 is devoted to the examination of the binding specificity and structural characteristics unique to KIR2DS4. Finally as an additional test of activating KIR function we will analyze the activating KIR of the squirrel monkey, the only other primate species with an increased proportion of activating KIR. Interspecies comparisons have been vital in the design of experiments testing the functional effects of variation and in Aim 2 we will analyze variation in other gene families of the leukocyte receptor complex (LRC), natural killer complex (NKC) and MHC in a panel of related primate species. The results of these studies will provide a basis for further functional exploration of differing immune responses among closely-related primate species. Aim 3 explores NK receptor variation and resolution of viral infection. We will test two hypotheses that can explain our observed correlation of activating KIR genotype and resolution of hepatitis C virus (HCV) infection in chimpanzees. In the second part of the aim the differences in variation and diversity of KIR, MHC, and other innate immune gene families between sooty mangabeys and rhesus macaques will be determined. These two species differ in their response to infection with simian immunodeficiency virus (SIVsm). This comparative analysis will identify unique species characteristics and the basis of differential SIV/HIV-2 immune responses.